Nitrogen is frequently prepared by the fractional distillation of liquid air or by pressure swing adsorption (PSA) from atmospheric air. In the synthesis of nitrogen by PSA, the feed air is compressed and/or a vacuum pump is employed for reduction of bed pressure. Nitrogen recovery by PSA is rather low (40-50%) and the mechanical power required is very high. In the air fractionation process, likewise, the feed air needs to be compressed (to about 100 psi) and cooled prior to distillation. Therefore, it is relatively expensive to isolate nitrogen by this method.